Add local CLI access via Unix-socket server

New CLIServerService listens on a Unix domain socket
(~/Library/Application Support/oAI/cli.sock) speaking a minimal
HTTP/1.1 subset, for one-shot non-streaming shell access to a single
fixed model — e.g. an `ai "prompt"` zsh function — without opening
the app window and without going through the tool-calling loop.

Configured in Settings > MCP > CLI Access (toggle, provider, model —
deliberately independent of the chat UI's active model). JSON
request/response envelope rather than raw text so new fields (model
override, streaming, tool support) can be added later without a
breaking wire-format change.

Verified live end-to-end against a real OpenRouter request, error
paths, and clean-shutdown socket cleanup. 10 new unit tests cover
the HTTP framing/parsing logic.

oAI/Utilities/Logging.swift

@@ -152,4 +152,5 @@ enum Log {
     nonisolated static let ui       = AppLogger(subsystem: subsystem, category: "ui")
     nonisolated static let general  = AppLogger(subsystem: subsystem, category: "general")
     nonisolated static let extMcp   = AppLogger(subsystem: subsystem, category: "ext-mcp")
+    nonisolated static let cli      = AppLogger(subsystem: subsystem, category: "cli")
 }